Organic light-emitting diodes (OLEDs) made of organic semiconductor materials have a great potential in the applications of novel optoelectronic devices such as in the applications of flat panel displays and lighting because of the synthetic diversity, low manufacturing cost, and high optical and electrical performance of organic semiconductive materials, making it possible to manufacture a large-scale flexible device. In order to improve the luminous efficiency of organic light-emitting diodes, various light emitter materials based on fluorescence and phosphorescence have been developed. The organic light-emitting diode of the fluorescent material has a high reliability; however, since the branching ratio of the singlet excited state and the triplet excited state of an exciton is 1:3, its internal electroluminescence quantum efficiency is limited to 25% under the electrical excitation. In contrast, the internal luminescence quantum efficiency of organic light-emitting diodes using phosphorescent materials has achieved almost 100%. So far, the phosphorescent materials which have practical value are iridium and platinum complexes; the cost is quite high since the raw material is rare and expensive and the synthesis of the complex is rather complicated.
In order to solve this problem, Adachi proposed the concept of reverse intersystem crossing so that an organic compound can be used, i.e. without using the metal complex, to achieve a high efficiency of phosphorescent OLED. Such a concept has come true by 1) an exciplex, see Adachi et al., Nature Photonics, Vol 6, p253 (2012); 2) thermal excited delayed fluorescent material TADF, see Adachi et al., Nature Vol 492, 234 (2012). But the OLED devices still have a very short life.
Obviously the efficiency and the life of the existing luminescent materials have yet to be improved.
Therefore, there is a need for improvement and development of the existing technology.